All living creatures are made of cells. One cellular component, the membrane, plays a crucial role in almost all cellular activities. The primary function of all cell membranes is to act as barriers between the intracellular and extracellular environments, and as sites for diverse biochemical activities. The cell itself is encapsulated by its own membrane, the plasma membrane. Although the composition of membranes varies, in general, lipid molecules make up approximately 40 percent of their dry weight; proteins, approximately 60 percent. The lipids and proteins are held together by noncovalent interactions.
Among several possible stable arrangements of protein and lipid molecules in membranes, the bilayer model, first described over seventy years ago, characterizes most biological membranes. An important feature of this model is that the hydrophilic groups of the lipid molecules are oriented toward the surfaces of the bilayer, and the hydrophobic groups toward the interior. In 1972 Jonathan Singer and Garth Nicolson postulated a unified theory of membrane structure called the fluid-mosaic model. They proposed that the matrix, or continuous part, of membrane structure is a fluid bilayer, and that globular amphiphilic proteins are embedded in a single monolayer, with some proteins spanning the thickness of both monolayers. Both proteins and lipids are mobile and, thus, the membrane can be viewed as a twodimensional solution of proteins in lipids.
The major class of lipids in plasma membranes is phospholipids. Phospholipids consist of a glycerol backbone and two fatty acids joined by ester linkage to the first two carbons of glycerol, and a phosphate group joined to the third. Different groups can be esterified to the phosphate, and these groups define the different classes of phospholipids. In addition, the fatty acids have varying chain lengths and degrees of unsaturation. The presence of the nonpolar acyl chain regions and the polar head groups gives the phospholipid molecules their amphipathic character, which allows them to assume the bilayer arrangements of membranes. In addition to phospholipids, two other kinds of lipids are found in the membranes of animal cells: glycolipids and cholesterol. Glycolipids usually make up only a small fraction of the lipids in the membrane but have been shown to possess many biological functions, one of which is their capacity to function as recognition sites. Cholesterol is an important component of plasma membranes and has been shown to play a key role in the control of membrane fluidity.
Several membrane functions are believed to be largely mediated by proteins. Membrane proteins have been put into two general categories: peripheral and integral. Peripheral proteins (or extrinsic proteins) are those that do not penetrate the bilayer to any significant degree and are associated with it by virtue of noncovalent interactions (ionic interactions and hydrogen bonds ) between membrane surfaces and protein surfaces. Integral proteins (or intrinsic proteins), in contrast, possess hydrophobic surfaces that readily penetrate the lipid bilayer, as well as other surfaces that prefer contact with aqueous medium. These proteins can either insert into the membrane or extend all the way across it and expose themselves to the aqueous solutions on both sides.
One of the main functions of the plasma membrane is to separate cytoplasm from extracellular surroundings. In fact, membranes are highly selective permeability barriers, as they contain specific channels and pumps that enable the transport of substances across membranes. These transport systems to a large degree regulate the molecular and ionic composition of intracellular media. Membranes also control the flow of information between cells, and between cells and their extracellular environments, and they contain specific receptors that make membranes sensible to external stimuli. In addition, some membranes conduct and pass on signals that can be chemical or electrical, as in the transmission of nerve impulses. Thus, membranes play a central role in signal transduction processes and in biological communication.
Glycolipids and glycoproteins can act as recognition sites in a variety of processes involving recognition between cell types or recognition of cellular structures by other molecules. Recognition events are important in normal cell growth, fertilization, transformation of cells, and other processes.
Garrett, Reginald H., and Grisham, Charles M. (2002). Principles of Biochemistry with a Human Focus. Fort Worth, TX: Harcourt College Publishers.
Harrison, Roger, and Lunt, George G. (1980). Biological Membranes: Their Structure and Function. New York: Wiley.
Singer, S. Jonathan, and Nicolson, Garth L. (1972). "The Fluid Mosaic Model of the Structure of Cell Membranes." Nature 175:720–731.